This invention relates to detectors of the ionization type for detecting airborne particulate matter and, in particular, to the construction of an ionization chamber for such a detector. The present invention will be discussed in terms of the application of such a detector for detecting combustion products such as smoke, but it will be understood that the invention could be used for detecting a variety of materials, such as dust, fog and the like.
Ionization-type smoke detectors are known and typically include an ionization chamber having two electrodes, means for establishing an electric field between these electrodes, and means such as a radioactive source for causing ionization within the chamber. This radiation produces ions in the chamber and the electric field creates an ion current flow between the electrodes. As combustion products enter the chamber, the ions attach themselves to these products and the magnitude of the ion current is accordingly reduced. The reduction in ion current amplitude is sensed by circuit means and when the current is reduced to a predetermined level, an electrical signal is generated which initiates a visible and/or audible alarm indication.
Prior ionization-type smoke detectors have exhibited an instability attributable to air currents which operate to trigger false alarms. More specifically, the airflow through the ionization chamber may carry some of the ions from the chamber and cause a reduction in quiescent ion current which triggers a false alarm. An attempt to correct this problem by the use of small or well-baffled air inlets effectively limits access to the ionization chamber of airborne combustion products, thereby reducing the sensitivity of the detector.
Attempts have also been made to produce an ionization-type smoke detector which is relatively insensitive to airflow velocity through the ionization chamber without unduly sacrificing sensitivity to airborne combustion products. One such attempt is disclosed in U.S. Pat. No. 4,185,196 in which the ionization source is designed so as to produce in the chamber a unipolar region containing charged particles of the same polarity. Furthermore, in that patent the electric field has high and low intensity regions arranged so that the airflow will carry ions from the low-intensity region to the high-intensity region so as to replace ions which are carried out of the chamber by air currents.
While this approach has been effective in reducing the adverse effects of air currents on the ionization chamber, it does so at the expense of a relatively closed chamber which has air inlet apertures at only one end thereof and, consequently, does not provide an efficient airflow path through the ionization chamber. As a result, the device of the '196 patent suffers from impaired sensitivity to combustion products, particularly the products of smoldering-type combustion.